1. Technical Field
The present invention relates to a propagation measuring apparatus and a propagation measuring method.
2. Related Art
Recently, the art of measuring propagation characteristics of an object in a terahertz light area (about 100 GHz˜10 THz) has been noted.
FIG. 9 shows a conventional propagation measuring apparatus 200 schematically. The propagation measuring apparatus 200 measures the propagation characteristics of an object by using femtosecond optical pulses, i.e., extremely short optical pulses whose pulse width is femtosecond-order. In addition, the art is written in, e.g., “The Frontiers of Information and Communication Research, All of Communications Reseach Laboratory” (The third chapter: The Exploitation of Frequency Resources, 10 Terahertz Electronics; Dempa Shimbun-sha).
In the propagation measuring apparatus 200, a femtosecond light pulse 110 of, e.g., less than 100 femtoseconds outputted from a pulse laser is split by a beam splitter 112. One of the femtosecond pulse 110 split by the beam splitter 112 is inputted into a terahertz electromagnetic wave transmitter 118 as a trigger pulse 114. The other one is inputted into a terahertz electromagnetic wave receiver 122 as a sampling pulse 116. The sampling pulse 116 is delayed properly by a time delay means 120.
The terahertz electromagnetic wave transmitter 118 is configured by forming a parallel transmission line 124 functioning as an antenna as well onto a light conductive film (not shown) made of low temperature grown gallium arsenide. The light conductive film made of low temperature grown gallium arsenide has excellent characteristics, in which the optical response speed is extremely high.
In a state where the terahertz electromagnetic wave transmitter 118 is applied with direct current voltage 126, when the trigger pulse 114 of, e.g., less than 100 femto seconds is incident into a gap part of the parallel transmission line 124, the parallel transmission line 124 is short-circuited temporarily, and thus a precipitous change in current occurs. Accordingly, terahertz pulses including frequency components of about 100 GHz to a few THz are transmitted from the antenna of the terahertz electromagnetic wave transmitter 118.
The terahertz pulse transmitted from the terahertz electromagnetic wave transmitter 118 is focused by off-axis parabolic mirrors 128 and incident into the sample 130. The terahertz pulse passing through the sample 130 is forced by off-axis parabolic mirrors 128 and incident into the terahertz electromagnetic wave receiver 122.
As the terahertz electromagnetic wave receiver 122, a device such as the terahertz electromagnetic wave transmitter 118 is used. The terahertz electromagnetic wave receiver 122 samples the terahertz electromagnetic wave incident into the gap part of the parallel transmission line 132 functioning as an antenna by using the sampling pulse 116. The signal obtained by the terahertz electromagnetic wave receiver 122 is amplified by a current amplifier 134.
The propagation measuring apparatus 200 detects the waveform of the terahertz pulse excited from the terahertz electromagnetic wave transmitter 118 while changing the delay time of the sampling pulse 116. In the propagation measuring apparatus 200 the waveform of the terahertz pulse is detected in relation to the delay time. The terahertz pulse detected in this way is coherent in a broadband and includes information relating to amplitude and phase, so it is useful for examining the physical characteristics of the sample 130. In addition, by measuring the sample 130 being moved in X-Y directions it is possible to visualize the physical characteristics of the sample.
In addition, a wavelength dispersion measuring apparatus for measuring and obtaining the wavelength dispersion of an object to be measured in regard to the close infrared light area is proposed.
FIG. 10 shows a conventional wavelength dispersion measuring apparatus 300 schematically. Further, the art of this is written in S. Ryu, Y. Horiuchi and K. Mochizuki, “Novel chromatic dispersion measurement method over continuous Gigahertz tuning range”, J. Lightwave Technol., Vol.7, No.8, pp.1177˜1180, 1989 or M. Fujise, M. Kuwazuru, M. Nunokawa and Y. Iwamoto, “Chromatic dispersion measurement over a 100-km dispersion-shifted single-mode fibre by a new phase-shift technique”, Electron. Lett., Vol. 22, No.11, pp.570˜572, 1986.
In the wavelength dispersion measuring apparatus 300, the laser light of the close infrared light area outputted from a wavelength variable light source 210 is inputted into an optical intensity modulator 212. The optical intensity modulator 212 modulates the intensity of the laser light outputted from the wavelength variable light source 210 with a sine wave by using a reference signal of frequency fIF supplied from a high frequency reference signal source 214.
The laser light modulated by the optical intensity modulator 212 is incident into an object to be measured 216. The laser light passing through the object to be measured 216 is detected by a photoelectric converter 218, converted into an electrical signal and amplified by an amplifier 220. The detected signal amplified by the amplifier 220 is inputted into a phase-amplitude comparator 224 of a network analyzer 222.
The phase-amplitude comparator 224 compares the detected signal amplified by the amplifier 220 with the reference signal supplied from the high frequency reference signal source 214. The comparison result by the phase-amplitude comparator 224 is converted by an A/D converter 226. And, a group delay time is obtained by a data processing block 228 as below.
The group delay time τ(λi) is represented by:τ(λi)=ø(λi,fIF)/(2πfIF),
wherein, the wavelength of the laser light outputted from the wavelength variable light source 210 is λi, the frequency of the reference signal supplied from the high frequency reference signal source 214 is fIF and the phase obtained by the phase-amplitude comparator 224 is ø(λi, fIF).
Therefore, by performing the measurement as above while changing the wavelength λi of the laser light outputted from the wavelength variable light source 210 continuously, the group delay time τ(λi) can be obtained for each wavelength. In addition, the wavelength dispersion D(λi), which is the differentiation of the group delay time τ(λi) with respect to the wavelength, is represented by:D(λi)=Δτ(λi)/Δλi,
wherein Δτ(λi)=τ(λi+1)−τ(λi) and Δλi=λi+1−λi.
In addition, the method for obtaining the group delay time τ(λi) as above is called the phase shift method.
In the wavelength dispersion measuring apparatus 300 it is possible to measure the group delay time τ(λi) by using the phase analysis art of high frequency, so that the measurement of the wavelength dispersion can be performed with high precision.
In addition, an apparatus for obtaining an optical tomographic image of an object to be measured with the heterodyne detection is proposed.
FIG. 11 shows a conventional apparatus for obtaining an optical tomographic image 400 schematically. Further, the art of this is disclosed in Japanese Patent Application Publications Nos. 1990-150747 and 2000-121550.
In the apparatus for obtaining an optical tomographic image 400, a laser light outputted from a laser light source 310 is focused by lenses 312 and 314 and split by a beam splitter 316.
One of the laser light split by the beam splitter 316 is irradiated to an object to be measured 318. The laser light passing through the object to be measured 318 is reflected by a mirror 320 and incident into a beam splitter 322.
The other laser light split by the beam splitter 316 is reflected by a mirror 324 and inputted into a light frequency converter 326. The light frequency converter 326 converts the frequency of the laser light by using a reference signal supplied from a high frequency reference signal source 328. The laser light whose frequency has been converted by the light frequency converter 326 is incident into the beam splitter 322.
In the beam splitter 322, the laser light passing through the object to be measured 318 and the laser light whose frequency has been converted by the light frequency converter 326 are merged. The laser light merged by the beam splitter 322 is incident into a light detector 324.
The light detector 324 detects the light by using the heterodyne detection. In the heterodyne detection, there is directivity, so that only the direct component of the laser light passing through the object to be measured 318 is detected.
The light detected by the light detector 324 is demodulated by a demodulator 326 and analyzed by a computer 329. The analysis result by the computer 329 is displayed as a tomographic image by an image display device 330.
The principle of the apparatus for obtaining an optical tomographic image 400 is shown below.
The electric field e1(t) of the light incident into the object to be measured 318 and the electric field e2(t) of the light outputted from the light frequency converter 326 are respectively represented by:e1(t)=A1 cos ωCtande2(t)=A2 cos {(ωC+p)t+θ2},
wherein A1 and A2 are the electric field strengths, p is the shift amount of the angular frequency of the light, θ2 is a constant phase and ωC is the angular frequency of the light.
In addition, the transfer function Y(ωC) of the object to be measured 318 is represented by:Y(ωC)=Y0exp(jø0),
then the electric field ed incident into the light detector 324 is represented by:ed=Y(ωC)A1 cos ωCt+A2 cos {(ωC+p)t+θ2} and
the output current id of the light detector 324 is represented by:id=α{Y02A12+A22+2Y0A1A2 cos(pt+θ2−ø0)},
wherein α is the proportional coefficient, ø0 is the phase and ωC is the angular frequency of the light. In addition, among the phase ø0, the angular frequency of the light ωC, the frequency the group delay time τ(ωC) there is a relation as follows:τ(ωC)=δø0/δωC.
According to the apparatus for obtaining an optical tomographic image 400, by the principle above it is possible to image the tomogram of the object to be measured.
However, in the propagation measuring apparatus 200 shown in FIG. 9, since the propagation characteristics of the sample 130 is measured by using the femtosecond light pulse alone, it is impossible to measure the frequency dependence of the propagation characteristics of the sample 130 in regard to the terahertz light area in detail. Moreover, in the propagation measuring apparatus 200 shown in FIG. 9, the light source generating the femtosecond pulse 110 is of a large scale, and it lacks the stability of, the pulse width or the optical intensity. Further, since the propagation characteristics of the sample 130 is measured while changing the delay time of the sampling pulse 116 gradually, it takes a long time to perform the measurement. In addition, due to using the spatial light, the apparatus becomes large and complicated, so it lacks the stability. Moreover, since the time delay means 120 changes the delay time by using a movable stage, it is affected by the mechanical precision of the movable stage and it is not necessary to be able to perform the measurement with high precision.
In addition, in case of applying the art of the wavelength dispersion measuring apparatus 300 shown in FIG. 10 to the terahertz light area, it is necessary to use a broadband optical intensity modulator of the terahertz light area as the optical intensity modulator 212. However, since the broadband optical intensity modulator of the terahertz light area is not widely spread, it is difficult to procure it and its price is high.
Moreover, in case of applying the art of the wavelength dispersion measuring apparatus 300 shown in FIG. 10 to the terahertz light area, the terahertz light outputted from the wavelength variable light source 210 is inputted into the photoelectric converter 218 via the optical intensity modulator 212 and the object to be measured 216. The transmission technique by which the loss in regard to the terahertz light area is low has not yet been progressed, it is necessary to make the intensity of the terahertz light outputted from the wavelength variable light source 210 large as much as the transmission loss. Therefore, it is necessary to use the wavelength variable light source 210 whose output is great, and thus it becomes an obstacle to making the apparatus small.
In addition, in case of applying the art of the apparatus for obtaining an optical tomographic image 400 shown in FIG. 11 to the terahertz light area, it is necessary to use a broadband light frequency converter of the terahertz light area as the light frequency converter 326. However, since the broadband light frequency converter of the terahertz light area is not widely spread, it is difficult to procure it and its price is high.
In addition, in case of applying the art of the apparatus for obtaining an optical tomographic image 400 shown in FIG. 11 to the terahertz light area, the terahertz light outputted from the laser light source 310 is inputted into the photoelectric converter 324 via the lenses 312 and 314, the beam splitters 316 and 322, the object to be measured 318, the light frequency converter 326, the mirrors 320 and 324. The transmission technique by which the loss in regard to the terahertz light area is low has not yet been progressed, it is necessary to make the intensity of the terahertz light outputted from the laser light source 310 large as much as the transmission loss. Therefore, it is necessary to use the laser light source 310 whose output is great, and thus it becomes an obstacle to making the apparatus small.
In addition, in case of applying the art of the apparatus for obtaining an optical tomographic image 400 shown in FIG. 11 to the terahertz light area, the path of the terahertz light lies on both the object to be measured 318 side and the light frequency converter 326 side, so the optical axis adjustment is complicated.
In addition, since the art of the apparatus for obtaining an optical tomographic image 400 shown in FIG. 11 is to measure only the amplitude without performing any phase comparison, the transmission delay time of the object to be measured 318 cannot be measured, and only the amplitude information such as transmission attenuation can be measured.